The Prior Art
4,4'-dihydroxybiphenyl is a well known compound useful as a starting material for a variety of products such as dyes and resins, e.g., polyesters, polyepoxides, polyurethanes, polycarbonates and high performance polymers. The process for the preparation of DHBP is described in Japan Kokai 79-22347 (Hodogaya). The product obtained according to this patent was of low purity and was not suitable for most applications. In a later patent of the same applicant (U.S. Pat. No. 4,340,768) an improved process is described in which the crude product, obtained according to the process described in the aforementioned Japan Kokai, is purified by extracting 4-hydroxybiphenyl (4-HBP) from the product mixture with a suitable solvent and the remaining product solution is neutralized, crystallized, filtered and dried, yielding a product of a purity greater than 98% and containing less than about 30 ppm of inorganic salts. This patent indicates that "it is impossible to purify 4,4'-dihydroxybiphenyl by a distillation so as to give the quality required for the demands in view of the characteristics of 4,4'-dihydroxybiphenyl".
An alternative process for obtaining DHBP of high purity is described in U.S. Pat. No. 4,490,564 (Ethyl Corporation). According to this patent, the crude product is dissolved in an inert organic solvent, preferably at elevated temperatures. Water is then added to the inert organic solvent mixture which allows the inorganic salts that may be present to remain in the solution and the product to precipitate out. Although this procedure is allegedly technologically superior to that described in U.S. Pat. No. 4,340,768, the results in the examples do not substantiate this allegation, with respect to the final purity obtained. Additionally, the procedure of this patent is cumbersome, involving operations with organic solvents, filtration and drying.
Additional patents dealing with the production of DHBP using DBBP as raw material do not address in detail the problem of the purity of the product and purification processes therefore. Among these patents there are found U.S. Pat. No. 4,475,000 (Ethyl Corporation), French Patent Application FR 2,609,711 (Rhone-Poulenc Chemie), and EP 278,845.